Goodwill and Other Intangible Assets - Summary of gross carrying amounts - (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other intangible assets
Gross carrying amount	$ 126,449	$ 82,154
Accumulated amortization	(52,660)	(42,306)
Net carrying amount	73,789	39,848
Amortization expense	10,353	7,577	$ 8,324
Estimated amortization expense for other intangibles for each of the next five years
2018	13,614
2019	12,585
2020	11,419
2021	10,186
2022	8,918
Thereafter	17,067
Total estimated amortization expense for other intangibles for the next five years	73,789
Amortization of Intangible Assets	$ 10,353	$ 7,577	$ 8,324
Minimum
Other intangible assets
Estimated useful lives	2 years
Maximum
Other intangible assets
Estimated useful lives	15 years